Sitting in airports for hours on end does offer time to read. The book is a collection of roughly 10-page essays about biology, specifically animal biology, and where we stood in 2014, when the book was written. Broadly focused on evolution, Wallace takes a sensible, textbook approach to the organization of the chapters. The first chapter is simply titled, 'What is an animal?' Obviously if you're going to discuss them, it would be helpful to define what they are. This is also a good way to establish some basic terminology about cell parts of body structures.
Wallace is writing for, I believe the layperson, or maybe a biology major just getting started. While he does occasionally get into the weeds on terminology - such as a discussion of one use of the term "recapitulation" in biology versus another - he tries to limit the use of geologic period names and larger family and class names. He doesn't avoid them entirely - I had never even heard of the Lophotrochozoa. It's a name for a group of protostomes (meaning their mouth develops before their anus, primates are in the other category) that include annelids and molluscs.
There's a lot of discussion of work done sequencing genes or embryology, using them to determine how long ago two groups branched from each other. Wallace contrasts the conclusions drawn from that evidence with past conclusions drawn when biologists didn't have DNA evidence. Under the latter, annelids were considered to be close to arthropods, because both have segmented bodies. The more recent evidence suggests roundworms (like tapeworms) are closer to arthropods.
Some parts are more interesting than others. The final chapter, attempting to discuss consciousness, how it's defined, when it might have developed, was so theoretical it didn't engage me much. I was more interested in the idea of evolutionary novelties, like a jellyfish's stinging cells or a turtle's shell, and the idea of at what level does a development cease to be a novelty? At one point in time, jaws would have been considered a novelty, but now they aren't. I wonder if opposable thumbs count?
Something I was familiar with, but still enjoyed, was the chapter on, essentially, how it's easier to lose a structure or adaptation than it is to gain it. In the book, the example is of mammals that adapted back to an aquatic life from terrestrial. While whales, seals, and manatees all returned to the sea from different lineages (whales seem like they're from the same line that produced even-toed animals like hippos, manatees maybe share ancestors with elephants, and seals are closest to bears, apparently), they developed roughly similar body types, and manatees and whales' feet turned into a flattened tail.
But while they regained some physical traits similar to fish (ancient fish being an ancestor to mammals), none of them have regained gills. Whales and dolphins developed a blowhole, but that's a modification of their existing respiratory system. Likewise, terrestrial snails developed from aquatic ancestors. Those terrestrial snails in some cases later developed into species to live in freshwater. None of those snails have developed gills either. An aquatic animal that develops from a terrestrial one might be able to recapture some morphology, but it seems like some structures are too complicated to get back once lost. Along those lines, I would imagine there aren't going to be species of snakes in the future that regain functional legs.
If evolutionary biology is something you have interest in, the book is worth a look. Wallace will also cite other biologists' work, which might focus more extensively on something you want to explore further.
'Perhaps more complex animals require not more genes but rather more complex patterns of gene-switching. In relation to this point, it is interesting to note a recent discovery made by American biologist Kevin Peterson and his colleagues. They studied a group of tiny molecules (called mico-RNAs) involved in gene-switching, and found that more complex animals had more families of micro-RNAs.'
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